This invention relates to nuclear fuel for liquid metal cooled nuclear reactors comprising a cluster of closely spaced parallel nuclear fuel pins shrouded by an annular wrapper. The cluster, its supporting means and the wrapper are commonly referred to as a fuel assembly and, in use, an assembly occupies, or defines, a coolant channel with the longitudinal axis of the cluster coincident with the axis of the channel. The elongated coolant flow paths between adjacent fuel pins are referred to as sub-channels and these sub-channels are conventionally long and narrow for nuclear physics reasons and to provide a compact core. In the unlikely event of a local restriction to flow occurring in one or more of the sub-channels, parts of the fuel pin surface lying in the wake of the restriction may be incompletely cooled. The consequential local temperature rise may have a deleterious effect on the fuel pin cladding. Such a local temperature rise may not be detectable early by the temperature sensitive transducer, which is located at the coolant channel outlet and senses an average value of outlet temperature. The latter is a parameter not affected by a local temperature rise because coolant in the blockage wake probably tends to form local eddies, decoupled from the main stream of the main coolant flow.
The aim of the present invention is to ensure that should a flow restriction develop then the resultant local temperature rise will be held at a safe value at least until the outlet temperature transducer responds to the defective condition.